Pour a few drops of oil into a square cake tin.Sprinkle the tin with a little flour. Set aside.Put the chocolate and butter into a heatproof bowl.Put a little water in a saucepan and heat gently until it simmers. Stand the bowl over the water making sure it does not touch the hot water.Turn the heat down to medium. Using a wooden spoon,stir the chocolate and butter until it has melted. Remove the pan and stir in the sugar ,keep on stirring ocasionally.Set aside.
Sieve the flour and salt together into a large mixing bowl. Break the egg into a small mixing bowl.Pick out any pieces of egg shell that have fallen into the bowl. Beat the egg lightly with a fork until the yolk and the white are mixed together.Add the vanilla extract and set aside.
On a chopping board,roughly chop the walnuts using a kitchen knife.Set aside.
Add the chocolate mixture,beaten eggs and walnuts to the flour.Mix well until smooth,using a wooden spoon.Spoon the mixture into the prepared tin and level the surface with a palette knife.
Place the tin in the oven and bake for 35-40 minutes or until the brownies are well risen.Wearing oven gloves, take the tin out of the oven. Leave the brownies in the tin for about 5-10 minutes or until almost cold. Using a kitchen knife, carefully cut the brownies into 12 bars.Carefully remove the brownies from the baking tin using a palette knife.